Starch synthesis occurs in the chloroplast while soluble carbohydrate (i.e., sucrose) synthesis occurs in the cytosol. These biosynthetic pathways are competing processes because excess triose phosphate can be used for either starch synthesis in the chloroplast or sucrose synthesis in the cytosol. These pathways have many common steps, however, the enzymes that catalyze similar steps are unique to each compartment. These enzymes are isozymes; different forms of the enzymes that catalyze the same reaction. For example, the plastidic and cytosolic forms of phosphoglucomutase both catalyze the conversion of glucose-6-phosphate to glucose 1-phosphate in different subcellular locations.
At maturity, about 40% of soybean seed dry weight is protein and 20% extractable oil. These constitute the economically valuable products of the soybean crop. Of the remaining 40% of seed weight, about 10% is soluble carbohydrate. The soluble carbohydrate portion contributes little to the economic value of soybean seeds and the main component of the soluble carbohydrate fraction, raffinosaccharides, are deleterious both to processing and to the food value of soybean meal in monogastric animals (Coon et al., (1988) Proceedings Soybean Utilization Alternatives, Univ. of Minnesota, pp. 203-211).
It may be possible to modulate the size of the starch and soluble carbohydrate pools in plant cells by altering the catalytic activity of specific enzymes in the starch and soluble carbohydrate biosynthetic pathways, such as phosphoglucomutase or one or both of the large and small subunits of ADP-glucose pyrophosphorylase (Taiz L., et al. Plant Physiology; The Benjamin/Cummings Publishing Company: New York, 1991). For example, during soybean seed maturation a large portion of the glucose which is converted to soluble carbohydrates (sucrose, raffinose and stachyose) during soybean seed maturation comes from the break down of a starch pool which was produced slowly during the primary growth phase. Elimination of this transient starch pool may be a strategy for diverting carbon away from the soluble carbohydrate components of dry soybean seeds (sucrose, raffinose and stachyose) and into the more economically desirable components such as oil and protein. This strategy may also be applicable to other plants such as corn, rice and wheat. Elimination of ADP-glucose pyrophosphorylase expression in developing maize embryos may decrease the production of transient starch in that tissue and lead to a concomitant increase in the oil content of the embryo [Singletary, G et al. (2001) U.S. Pat. No. 6,232,529].
There is a great deal of interest in identifying the genes that encode proteins involved in starch and soluble carbohydrate biosynthesis in plants. The genes that code for these enzymes may be used to study the interactions among individuals of the pathways and develop methods to alter starch and soluble carbohydrate biosynthesis. Accordingly, the availability of nucleic acid sequences encoding all or a substantial portion of a plastidic or cytosolic phosphoglucomutase (PGM) enzyme would facilitate studies to better understand starch and soluble carbohydrate biosynthesis in plants and provide genetic tools to enhance or otherwise alter starch and soluble carbohydrate biosynthesis.
Previous reports on a plastidic PGM mutant (pgm-1) from the oilseed plant Arabidopsis (Caspar et al. (1985) Plant Physiol. 79:11-17; Periappuram et al., (2000) Plant Physiol. 122:1193-1199) indicated that pgm-1 mutant plants showed a decrease in seed lipid content and an increase in leaf soluble carbohydrates. High levels of soluble carbohydrates were also observed in starchless Nicotiana sylvestris plants deficient in the plastidic PGM activity (Huber and Hanson, (1992) Plant Physiol. 99:1449-1454). Yet another effect of reduced starch content on carbon partitioning was observed in pea (Pisum sativum). Seeds from wild type pea typically contain 60% of the seed dry weight as starch. The rug3 locus of Pisum sativum encodes the pea plastidic phosphoglucomutase. Pea seeds, of the rug3rug3 genotype, substantially lacking plastidic phosphoglucomutase activity, have a wrinkled phenotype, higher levels of sucrose and an increased lipid content at maturity (EP 1001029A1; Casey et al., (1998) J. Plant Physiol. 152: 636-640).